I started my novel in third person, past tense, then I changed it to third person present tense, then I changed it to first person present tense, and now I've changed it to first person past tense. I think this is the most comfortable. Are there any guidelines to subject matter/time setting of the novel, and so on, that I should know?
The predominant rule for writing applies to your question. Avoid confusing your reader.
If your story is changing POV and shifting in time, both to the past and to the present, the important element of writing is to make it clear the when, where, and who (POV) of each scene.
This can be done by establishing some well identifiable object or place or character with each time frame. Like a new red ball in the past, and a dull cracked faded red ball in the current time frame. When the scene changes to the past, have the new red ball somehow involved. Similarly, the old worn ball frames the present time. Obviously, a ball is a really bad example except for a very few kind of stories.
The writer Alice Munro makes excellent use of time shifts in her writing, if you want to read a master of that aspect of the craft.
And, there are other ways. Some stories explicitly call out time, date, and location of the scenes to alert the reader that the focus of the story has changed. It's one method that doesn't require as much skill to use, but it works well for certain kinds of stories since it lends an epistolary feel to the writing.
But, in the wrong kind of story, it can get old and annoying to the reader since its an obvious intrusion to the writing.
And, that is probably the second rule, Don't annoy your reader by switching POV and time frames just to make writing the story easier. You should do it to make the story better and more engaging and more suspenseful, but when the story is finished, the techniques and devices used should be seen as intrinsic to telling the best story, and not crude attempts to obfuscate and confuse the reader to the ending comes as a surprise.